In the field of electronics, there are many applications for electrically connecting a small diameter wire to a substantially planar surface. For example, one such application is the use of a wire to electrically interconnect a terminal pad of a microelectronic device such as an integrated circuit silicon chip to a conductor or substrate circuit of a package on which the chip is mounted.
Two common techniques of electrically connecting a wire to a substantially planar surface are wedge bonding and ball bonding. In wedge bonding, a wedge shaped ultrasonic tool is used to connect the outer cylindrical surface of the wire to the planar surface. Accordingly, the take-off angle of the wire from the planar surface is not perpendicular to the surface. In ball bonding, a spherical shape or ball is formed at the end of the wire and then the ball is bonded to the planar surface by a well-known method such as thermocompression or ultrasonic bonding. Generally, before bonding, the wire having a spherical shape or ball on the end is positioned in the capillary of a ball bonding tool which is then moved until the ball contacts the planar surface. After the bond is made, the wire take-off angle is perpendicular to the planar surface.
Ball bonding has advantages over wedge bonding. For example, because the take-off angle of the ball bond wire is perpendicular to the planar surface, the area of the bond itself is smaller than with a wedge bond. Furthermore, ball bonds can be made closer to pads, other bonds, terminals, components, conductors, or other structures with which electrical contact with the wire would be undesirable. Stated differently, ball bonds can be used in a more densely packed configuration. Also, because of the shape of the tools generally used for both types of bonds, the ball bond can be placed closer to a package side wall than a wedge bond. Ball bonding has the further advantage of being less expensive due in part to the fact that less operator time is required because the bonds can be made faster. In ball ball bonding, the operator only manipulates the tool in the X and Y direction; in wedge bonding, the angle of the bond may also be a critical factor to prevent wire contact with other structures.
Ball bonding, however, has had a disadvantage that wedge bonding does not have. That is, while wedge bonding has been performed using both gold and aluminum wires, ball bonding with aluminum has heretofore been impractical. Because of the advantages of ball bonding such as those related to the bond geometry, it has been most important to develop a method of ball bonding aluminum wires. The advantages of using aluminum wires over gold are that aluminum is much less expensive and exhibits the radiation hardening properties that may be required in military semiconductor applications. Also, a monometallic aluminum joint between aluminum pads and the wire is desirable because it eliminates the possibility of brittle intermetallic compounds which result in decreased reliability. In short, the desirability of being able to ball bond aluminum wires has been recognized for some time.
The reason that aluminum wire ball bonding has been impractical is the inability to form the spherical shape or ball on the end of the wire. With a gold wire, heat applied to the end of the wire by such well-known techniques as a hydrogen flame or spark discharge liquifies the end region and a ball forms as the result of the surface tension forces. With aluminum, however, an oxide layer on the wire inhibits the surface tension forces from forming the ball. The oxide layer forms naturally and very rapidly on an exposed surface of aluminum. Accordingly, even if the oxide layer is removed, new oxide rapidly forms to either prevent the formation of the ball or cause an irregular shape if one is formed.
Prior art attempts to develop a method for forming spherical ends on aluminum wires for ball bonding having included various heat sources such as, for example, a focused laser beam, a microplasma torch, plasma discharge, and a miniature radiant heater. Furthermore, heating operations have been carried out in a protective atmosphere such as in an inert gas to prevent the additional formation of oxide. However, it was found that the surface oxide film was sufficient to oppose the native surface surface tension forces and prevent the forming of a spherical end of quality required for ball bonding.
It has been found that better quality aluminum spheres can be formed by positioning the wire in an inert gas such as helium and using an electrode of much lower voltage than commonly used to generate the heat through an electric arc. For example, the arcing electrode for gold wires is typically about 1200 volts. By reducing the voltage of the electrode to about 500 volts, improved sphere formation on aluminum wires results. However, it is well known that the arc voltage must be in excess of the minimum sparking potential in the atmosphere of the spark gap. With the 1200 volt potential commonly used for gold, gaps on the order of 0.005 inches can be ionized and this gap spacing can be readily provided by existing equipment. However to obtain reliable ionization with a 500 volt potential, the acceptable gap has to be in the range from 0.001 to 0.002 inches. With existing equipment, there is a significant problem in obtaining these precise arc gaps. If the gap is too large, there is no ionization and no spark. One prior art method to overcome the small arc gap tolerance problem was to use a contact method of spark initiation. With potential voltages of less than 200 volts, the wire and/or electrode are moved until there is contact. This method, however, introduced new problems related to the precision of moving the small diameter wire in contact with the electrode to strike the arc.